VLSI CRTC's like the 6845 had a light pen input and a register that captured the character position at the time it fired. You could read that and translate it.

VLSI CRTC's like the 6845 had a light pen input and a register that captured the character position at the time it fired. You could read that and translate it. It was a small matter of integration to make the input work. This chip was meant for text display but the PC and other systems used it for pixel-based displays too.